Originally Posted by Dr. ANo Levels: It seems to be a different way of improving your character. Details are rather sketchy on this as well. All they've pretty much said is that they are are going for horizontal rather then vertical progression.

Classes have tiers, but these are not raised with XP or killing mobs. From what I can deduce of interviews and the class panel, you have to perform "class challenges" to be able to advance a tier. The devs didn't call them class challenges, but that's the closest I came up with to "do various things to get to the next tiers". I even think that these "challenges" are given to you by the class trainer, this is why you can't advance a Paladin class if you are doing evil things. You lose access to your trainer.

and the horizontal progression is collecting more classes and abilities. While the vertical one is raising your class tier, but the way they are talking there are only 4-5 of them.

Also, while you can learn lots of classes. You have to be "in a specific class" when playing and this will define what armor and weapons you can use and the type of class ability slots you have (different for all classes, but there are only 24 combos and over 40 classes, so they are redundancy). The slot types are movement (leap, dash, teleport, etc), offensive (damage dealing stuff like a fireball or mana burns), defensive (spell reflect, shielding, etc), utility (unsummon spell is the example they used).